In a printing press several plates are generally tensioned next to each other and one behind the other on an impression cylinder. Dummy plates are offset plates without ink-accepting parts. They are used when the paper web must not be completely printed. A dummy plate is tensioned on the impression cylinder there where no image may be transferred onto the paper. The objective of the dummy plate is to take up the fountain solution applied over the entire cylinder width and to transfer it to the paper web. At the same time the dummy plate may not accept the printing ink in order to avoid a transfer of the latter onto the paper ("toning"). A lot of dummy plates are necessary when separate inks in multicolour offset printing are not used on all of the pages so that the respective inking units may not transfer ink there.
Dummy plates are normally roughened and anodically oxidized aluminium plates. They can optionally have an extra hydrophilizing layer. The disadvantage of these dummy plates is that they are very sensitive to fingerprints and to other external influences. Fingerprints for example lead to ink acceptance and consequently to toning in the printing press. Furthermore it has been established that the hydrophilic characteristics of such dummy plates deteriorate upon ageing as a result of which they accept ink in the printing press and tone. This phenomenon is strongly present when the dummy plates were exposed without any protection to the ambient air for a prolonged period of time, which allowed them to take up e.g. moisture, dust or traces of oil. In order to eliminate the consequences of fingerprints and to restore the hydrophilic characteristics of the aluminium surface to the original value, dummy plates are subjected to the normal development process with subsequent gumming. Through development the printing-plate surface is cleaned and hydrophilized. A subsequent gumming preserves the surface. The tendency towards toning in the printing press is considerably less for dummy plates having received such a treatment.